1. Field of the Invention
This invention relates generally to the structure and fabrication process of trenched DMOS power transistors. More particularly, this invention relates to a novel and improved structure and process for fabricating trenched DMOS power device. This device is provided with flexibly adjustable threshold voltage and punch through prevention by adjusting the doping concentration of a body-dopant redistribution-compensation region under a source region near a vertical channel along the trenched gate of the DMOS transistor cells.
2. Description of the Prior Art
A trenched power DMOS device may be troubled by a punch through problem generated due to an anomaly encountered during silicon diffusion. The anomaly is a phenomenon recognized as the body dopant redistribution. The redistribution phenomenon is characterized by a "distribution coefficient m" representing the ratio of the equilibrium concentration of the impurity in silicon to its equilibrium concentration in the oxide for the impurity in a Si--SiO2 system. When the distribution coefficient m for a particular impurity is less than one, then a process of growing an oxide layer drives the impurity from the silicon region to the silicon dioxide region. The values of the distribution coefficient m for boron are function of crystal orientation. At typical diffusion temperature, the value of m ranges from 0.15 to 0.3 for {100} silicon. Due to the fact that the distribution coefficient m is smaller than one, FIGS. 1A and 1B shows the distribution of the boron concentration in the interface of the silicon and the oxide. The redistribution of the boron, i.e., the equilibrium concentrations, when a slow diffusion process is applied is even more pronounced as that shown in FIG. 1A than that of a slow diffusion shown in FIG. 1B.
Due to the redistribution of the boron ions when growing the gate-oxide, the body dopant concentration near the trenched gate is affected as that shown in FIGS. 2A and 2B. FIG. 2A is a cross sectional view of a trenched DMOS power device and FIG. 2B shows the net dopant concentration along two vertical line Y-Y' and Z-Z'. It is clear that the body dopant concentration near the trenched gate Y-Y' is lower than that along the line Z-Z' by a redistribution reduction .delta.P. With a low body dopant concentration near the trenched gate and the channel region, an early punch through is more likely to occur due to insufficient body dopant concentration near the channel regions.
Therefore, a need still exits in the art of power device fabrication, particularly for trenched DMOS design and fabrication, to provide a structure and fabrication process that would resolve the difficulties caused by the body dopant redistribution. More specifically, it is preferably that reduction of the body dopant concentration caused by this dopant redistribution can be properly compensated without requiring application of additional masks or complicated fabrication processes. Furthermore, it is desirable in compensating the dopant concentration, the threshold voltage can be flexibly controlled by adjusting the body dopant concentration near the channel region next to the trenched gate.